Reviewers: This article was reviewed by Lakshminarayan M. Iyer and I. King Jordan. For complete reviews, see the Reviewers’ Reports section.Polintons (also known as Mavericks) and Tlr elements of Tetrahymena thermophila represent two families of large DNA transposons widespread in eukaryotes. Here, we show that both Polintons and Tlr elements encode two key virion proteins, the major capsid protein with the double jelly-roll fold and the minor capsid protein, known as the penton, with the single jelly-roll topology. This observation along with the previously noted conservation of the genes for viral genome packaging ATPase and adenovirus-like protease strongly suggests that Polintons and Tlr elements combine features of bona fide viruses and transposons. We propose the name ‘Polintoviruses’ to denote these putative viruses that could have played a central role in the evolution of several groups of DNA viruses of eukaryotes.